The present invention relates to a liquid for producing a marker vapour, a method of producing a marker vapour with the liquid, and a method of inspection with marker vapour produced from the liquid
U.S. Pat. No. 5,107,698 (Gilliam) discloses a smoke generating apparatus used for leak detection. What is described as a xe2x80x9cfireproof hydraulic fluidxe2x80x9d is splashed onto a heating element. Upon contact with the heating element the hydraulic fluid is vapourized with incomplete combustion causing smoke as a byproduct. The smoke serves as a marker vapour as it exits pin sized holes that are causing leaks. This type of marker vapour is an aerosol, as it consists of a plurality of particles dispersed in a gas.
Toxicology reports on hydraulic fluid, and the smoke produced thereby, indicate potential harm to humans. It is, therefore, preferable that a switch be made to less toxic mediums. Experiments have been made with visible vapours. Visible vapours are gaseous forms of a normally liquid or solid substances. However, it has been determined that as pressure increases the visible vapours are no longer visible. The pressure at which the visible vapours are no longer visible is in a range of 30 to 50 psi, depending upon the lighting conditions under which the visible vapour is being viewed and the exit velocity of the visible vapour.
The term xe2x80x9cmarker vapourxe2x80x9d will hereinafter be used in its broadest sense of a substance diffused or suspended in air and will, therefore, encompass both aerosols and visible vapours.
What is required is a liquid suitable for use in producing a marker vapour, a method of producing a marker vapour with such a liquid, and a method of inspection with marker vapour produced from the liquid.
According to one aspect of the present invention there is provided a liquid for producing a marker vapour. The liquid includes a fluorescent substance in solution in a carrier liquid. The fluorescent substance has a first vapourization temperature range at which the fluorescent substance vapourizes. The carrier liquid has a second vapourization temperature range at which the carrier liquid vapourizes. The second vapourization temperature range overlaps the first vapourization range.
The liquid, as described above, produces a marker vapour that is visible at low pressure. At high pressure the marker vapour becomes visible when exposed to radiation of suitable wavelength. It is believed that there is a pressure drop as the marker vapour exits any vessel that is being checked for leaks. This pressure drop causes the marker vapour to experience a change in state from vapour to liquid, thereby depositing a fluorescent marker at the exit point. This enables anomalies or defects resulting in leakage to be clearly discernable upon inspection under radiation of suitable wavelength.
Once the concept of a vapour that contained a fluorescent marker was conceived, difficulties were experienced in putting the theory into practice. A number of fluorescent marker liquids existed that were used for non-aerosol applications. They consisted of a fluorescent substance in solution in a solvent based or water based carrier liquid. These existing fluorescent marker liquids proved not to be suitable for aerosol application, as the application of heat tended to separate their constituents. When a solvent based fluorescent marker liquid was exposed to vapourizing heat, the solvent tended to flash off, leaving the fluorescent substance behind. When a water based fluorescent marker liquid was exposed to vapourizing heat, the water tended to evaporate, leaving the fluorescent substance behind. Success was achieved by matching a fluorescent substance with a carrier fluid that had overlapping vapourization temperatures.
Although beneficial results were obtained through the use of the liquid for producing a marker vapour, as described above, it was discovered that the most effective vapourization temperature ranges for the carrier liquid frequently resulted in inefficient vapourization or even burning of the fluorescent substance, or vice visa. The fluorescent substance has a first critical point at which the liquid and vapour phases of the fluorescent substance are in equilibrium. The carrier liquid has a second critical point at which the liquid and vapour phases of the carrier liquid are at equilibrium. Even more beneficial results were obtained when the first critical point and the second critical point were substantially the same. This enabled a balancing of vapourization temperatures to be performed to efficiently vapourize both the fluorescent substance and the carrier liquid, without concern that accidental temperature fluctuations will result in combustion of one of the fluorescent substance or the carrier liquid. The process can be controlled to at all times maintain the temperature in the more efficient vapourization ranges and well below the combustion temperatures.
Although beneficial results may be obtained through the use of the liquid for producing marker vapour, as described above, it is preferred that the marker vapour be not only less harmful, but completely harmless. Even more beneficial results may, therefore, be obtained when both the fluorescent substance and the carrier liquid are non-toxic. There are a variety of non-toxic food grade oils that are suitable for use. There are also a variety of non-toxic fluorescent substances presently used in medical applications that are suitable.
After a series of unsuccessful experiments using water and various solvents as carrier liquids, beneficial results were first obtained using a non-toxic mineral oil and also using glycerine. It will be appreciated that it should be possible to use a variety of carrier liquids, including water or solvent. The key to developing such liquid for producing marker vapour lies in finding a fluorescent substance that has a similar vapourization temperature range as water or the particular solvent selected.
According to another aspect of the present invention there is provided a method of producing a marker vapour which includes the steps of providing a fluorescent marker liquid as described above and vapourizing the fluorescent marker liquid at a temperature that is within both the first vapourization temperature range and the second vapourization temperature range. This forms a vapour that is visible at low pressure, and becomes visible at high pressure when exposed to radiation of suitable wavelength.
Although beneficial results may be obtained through the use of the method, as described above, of the various ways of vapourization, the best results were obtained when the fluorescent marker liquid was vapourized by atomizing the fluorescent marker liquid onto a heated substrate.
According to another aspect of the present invention there is provided a method of inspection with marker vapour which includes the following described steps. A first step involves providing a fluorescent marker liquid consisting of a carrier liquid containing a fluorescent substance. A second step involves vapourizing the marker liquid to produce a vapour. A third step involves directing the vapour into a pressure container being inspected. A fourth step involves inspecting the pressure container under radiation of suitable wavelength to cause the fluorescent substance to fluoresce.